Rendre un TForm transparent en utilisant les Region

private
{ Private declarations }
FullRgn, ClientRgn, CtlRgn : THandle;
procedure MakeTransparent;
procedure UndoTransparent;
end;

{...}

implementation

{...}

procedure TForm1.MakeTransparent;
var
AControl : TControl;
A, Margin, X, Y, CtlX, CtlY : Integer;
begin
Margin := ( Width - ClientWidth ) div 2;
FullRgn := CreateRectRgn(0, 0, Width, Height);
X := Margin;
Y := Height - ClientHeight - Margin;
ClientRgn := CreateRectRgn( X, Y, X + ClientWidth, Y + ClientHeight );
CombineRgn( FullRgn, FullRgn, ClientRgn, RGN_DIFF );
for A := 0 to ControlCount - 1 do begin
AControl := Controls[A];
if ( AControl is TWinControl ) or ( AControl is TGraphicControl )
then with AControl do begin
if Visible then begin
CtlX := X + Left;
CtlY := Y + Top;
CtlRgn := CreateRectRgn( CtlX, CtlY, CtlX + Width, CtlY + Height );
CombineRgn( FullRgn, FullRgn, CtlRgn, RGN_OR );
end;
end;
end;
SetWindowRgn(Handle, FullRgn, TRUE);
end;

procedure TForm1.UndoTransparent;
begin
FullRgn := CreateRectRgn(0, 0, Width, Height);
CombineRgn(FullRgn, FullRgn, FullRgn, RGN_COPY);
SetWindowRgn(Handle, FullRgn, TRUE);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
MakeTransparent
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
UndoTransparent
end;